The present invention relates to an objective lens driving apparatus, and more particularly to an objective lens driving apparatus provided in an apparatus for optically recording and reproducing information by a light spot irradiated on a disc-shaped recording medium.
An objective lens driving apparatus is used in an optical disc drive which records and reproduces information represented by sequence of pits by irradiating a light beam spot on a disc-shaped information recording medium (hereinafter is referred to as disc) such as a compact disc. The objective lens driving apparatus drives an objective lens so that the light beam spot is accurately applied to a predetermined position on the disc. A focusing error and a tracking error ordinarily arise between the pit sequence on the rotating disc and the light beam spot. The focusing error is caused by vibration of a disc face in the axial direction (hereinafter is referred to as face vibration) and the tracking error is caused by eccentricity of the disc. To correct these errors, the objective lens driving apparatus controls the objective lens in a direction perpendicular to the disc face (hereinafter is referred to as focusing direction) as well as in the radial direction of the disc (hereinafter is referred to as tracking direction) so that an adequately focused light beam spot continues to trace accurately the pit sequence.
In CD-ROM drives, DVD-ROM drives, etc. used with personal computers, the increase of a data transfer speed is required with the improvement of processing performance of the computer. To increase the data transfer speed, it has been practiced to increase the disc rotational speed, thereby realizing a high speed recording and reproducing operation. However, with the increase of the disc rotational speed, the objective lens is required to be controlled at higher speeds to follow with vibrations caused by the face vibration varying at a high speed and disc eccentricity. In order to reply to the above-mentioned demand, a high acceleration sensitivity is required in the objective lens driving apparatus. The acceleration sensitivity is defined as the ratio of a current supplied to the objective lens driving apparatus to an acceleration when the objective lens moves. It is known that the acceleration sensitivity required to the objective lens driving apparatus increases with the square of the revolution speed of the disc.
In order to increase the acceleration sensitivity in the objective lens driving apparatus, the moving member on which the objective lens is mounted has been reduced in weight, and improvements are made in the magnetic circuit used to generate the driving force. A prior art objective lens driving apparatus will be described below with reference to drawings.
FIG. 12 is a perspective view of relevant parts of the prior art objective lens driving apparatus.
In FIG. 12, an objective lens 101 and a printed coil board 104 are rigidly fixed to a lens holder 102 to construct a moving member 100. Suspension wires 103a, 103b, and 103c, and a suspension wire 103d located below the suspension wire 103c and hidden from view in the drawing, are each fastened at one end to the lens holder 102 and at the other end to a wire holder 111. The wire holder 111 is fixed to a base 110. Two yoke bases 107a and 107b are mounted on the base 110 in opposed relationship. Magnets 108a and 108b are mounted on the opposing surfaces of the yoke bases 107a and 107b, respectively, thereby forming a magnetic circuit. The printed coil board 104 is located between the magnets 108a and 108b. 
FIG. 13 is a plan view of the printed coil board 104 and the magnets 108a and 108b, as seen in the direction of arrow V in FIG. 12. In FIG. 13, one focusing coil 105 and four tracking coils 116 to 119 are mounted on the printed coil board 104. Arrow Fo in the figures indicates the moving direction of the printed coil board 104 for focusing action (hereinafter is referred to as the focusing direction), and arrow Tk indicates the moving direction of the same for tracking action (hereinafter is referred to as tracking direction).
In FIG. 13, the focusing coil 105 substantially rectangular in shape is located in the central portion of the printed coil board 104. A current flows through the focusing coil 105 in the direction shown by arrow If (hereinafter is referred to as current If). Two tracking coils 116 and 117 substantially rectangular in shape are located at the left side of the focusing coil 105, and two tracking coils 118 and 119 substantially rectangular in shape are located at the right side thereof. The four tracking coils 116 to 119 are wired so that a current flows in the direction indicated by arrow It (hereinafter is referred to as current It). The printed coil board 104 includes one layer or a plurality of layers. When the printed coil board 104 includes a plurality of layers, the focusing coil 105 and the tracking coils 116 to 119 are respectively wired so that the current flows in the same direction in any of the coils on the respective layers.
In FIG. 12 and FIG. 13, the dimensions of the magnets 108a and 108b are determined so that their left and right sides align with the centers of the left-side tracking coils 116, 117 and the right-side tracking coils 118, 119, respectively. The vertical dimension of each of the magnets 108a and 108b is made larger than the vertical dimension of any of the focusing coil 105 and the tracking coils 116 to 119. The magnets 108a and 108b are magnetized so that each magnet has one magnetic pole (for example, N pole) on the surface thereof facing to the printed coil board 104 and the opposite magnetic pole (for example, S pole) on the opposite surface thereof. In FIG. 13, on the upper half sections of the magnets 108a and 108b sectioned by a magnetization boundary line MB of a horizontal dashed line; the magnetic lines of forces are directed from a viewer of the figure into behind the paper face of the figure, and in the lower half section, the magnetic lines of forces are directed from behind the paper face of the figure toward the viewer. The magnetization boundary line MB indicates that the magnetization state is reversed across the boundary line. The magnets 108a and 108b are magnetized so that the opposite magnetic poles face each other with the printed coil board 104 interposed therebetween.
When the current If flows through the focusing coil 105, winding portions 105a of the focusing coil 105 on two sides perpendicular to the focusing direction Fo receive an electromagnetic force in the focusing direction Fo in accordance with Fleming""s rules. As a result, the moving member 100 is driven in the focusing direction Fo. In winding portions 116a to 119a and 116b to 119b of the tracking coils 116 to 119 on respective two sides perpendicular to the tracking direction Tk, the winding portions 116b to 119b on the sides nearer to the focusing coil 105 are located within the magnetic field of the magnets 108a and 108b. When the current It flows through the tracking coils 116 to 119, the winding portions 116b to 119b receive the electromagnetic force in the tracking direction Tk in accordance with Fleming""s rules, and the moving member 100 is driven in the tracking direction. Improvements are intended for focusing drive sensitivity and tracking drive sensitivity by placing the printed coil board 104 within the magnetic field of high magnetic flux density formed by the two opposing magnets 108a and 108b. Where, the focusing drive sensitivity is defined as the ratio (LF/If) of a moving distance LF of the moving member 100 in the focusing direction Fo to the current If flowing through the focusing coil 105, and the tracking drive sensitivity is defined as the ratio (LT/It) of a moving distance LT of the moving member 100 in the tracking direction Tk to the current It flowing through the tracking coils 116 to 119.
In the above-mentioned configuration, in the focusing coil 105, the winding portions 105a on two of the four sides generate the driving force in the focusing direction Fo. In the tracking coils 116 to 119, on the other hand, only the winding portions 116b to 119b on one side of each tracking coil generate the driving force in the tracking direction Tk. As a result, the tracking drive sensitivity is lower than the focusing drive sensitivity, resulting in the problem that the moving member 100 cannot achieve a sufficient acceleration sensitivity necessary for high speed record and reproduction. Where, the acceleration sensitivity is defined as the ratio, xcex1/It or xcex1/If, of the acceleration xcex1 when the moving member 100 moves in the tracking direction or focusing direction to the current I. Another problem is that a tilt occurs in the moving member 106 because of the influence of moments associated with those portions (hereinafter is referred to as ineffective portions) of the tracking coils 116 to 119 which do not contribute to the generation of the driving force in the tracking direction Tk. This tilt problem will be described with reference to FIG. 14A and FIG. 14B.
FIG. 14A and FIG. 14B are plan views of the printed coil board 104 and the magnets 108a and 108b, as seen in the direction of arrow V in FIG. 12. In the figures, oblique hatching indicates the ineffective portions 116c, 117c, 118c, 119c, 116d, 117d, 118d, and 119d on two sides perpendicular to the focusing direction Fo, of the respective tracking coils 116 to 119 placed within the magnetic field of the magnets 108a and 108b. The moments about the center point O of the printed coil board 104, caused by the electromagnetic forces in the focusing direction Fo acting on the ineffective portions 116c to 119c and 116d to 119d, are indicated by arrows N1 and N2.
FIG. 14A shows the condition in which the current It flows through the tracking coils 116 to 119 and the printed coil board 104 is caused to move relative to the magnet 108a by a distance X in the tracking direction Tk. For simplicity, it is provided that the respective area-ratios between the ineffective portions 116c, 117c, 116d, 117d and the ineffective portions 118c, 119c, 118d, 119d are 2:1. It is also provided that the electromagnetic force acting points of any two ineffective portions positioned symmetrically about the center point O are located at approximately equal distances from the center point O. In the figure, the factor attached to letter xe2x80x9cexe2x80x9d indicates the magnitude of the electromagnetic force, and the arrow alongside it indicates the direction of the electromagnetic force. In the tracking coil 116, the upper and lower ineffective portions 116c and 116d receive electromagnetic forces 2e equal in magnitude but opposite in direction, and therefore the electromagnetic forces are cancelled. In the tracking coil 117 also, the upper and lower ineffective portions 117c and 117d receive electromagnetic forces 2e equal in magnitude but opposite in direction, and therefore the electromagnetic forces are cancelled. Likewise, in the tracking coils 118 and 119, the respective ineffective portions 118c and 118d or 119c and 119d receive electromagnetic forces e equal in magnitude but opposite in direction, and therefore the electromagnetic forces are cancelled. As a result, the clockwise moment N1 and counterclockwise moment N2 about the center point O become equal to each other, and the difference N between is zero.
Description is made as to the case where the printed coil board 104 is caused to move by the distance X in the tracking direction Tk and, at the same time, is caused to move by a distance Y in the focusing direction Fo by the application of the focusing current If, as shown in FIG. 14B. In this case, differences occur in the density of the magnetic flux passing through the ineffective portions 116c to 119c and 116d to 119d of the respective tracking coils 116 to 119. The reason is that, when viewed along the focusing direction Fo, the magnetic flux density of the magnetic field formed by the magnets 108a and 108b has a nonuniform distribution such that the flux density is highest near the center CP (indicated by semi-dashed line) of each magnetic pole and lowest near the magnetization boundary line ML indicated by dashed line and also near the edge E.
The case that the printed coil board 104 is moved as illustrated in FIG. 14B will be described by using specific numerical values. The ineffective portions 116c and 118c of the tracking coils 116 and 118 move closer to the edge E where the magnetic flux density of the magnets 108a and 108b is low. Assume that the electromagnetic forces have therefore decreased by 20% to 1.6e and 0.8e, respectively. On the other hand, the ineffective portions 116d and 118d of the tracking coils 116 and 118 move closer to the center CP where the magnetic flux density of the magnets 108a and 108b is high. Assume that the electromagnetic forces have therefore increased by 20% to 2.4e and 1.2e, respectively. Further, the ineffective portions 117c and 119c of the tracking coils 117 and 119 move closer to the magnetization boundary line ML where the magnetic flux density of the magnets 108a and 108b is low. Assume that the electromagnetic forces have therefore decreased by 20% to 1.6e and 0.8e, respectively. On the other hand, the ineffective portions 117d and 119d of the tracking coils 117 and 119 move closer to the center CP where the magnetic flux density of the magnets 108a and 108b is high. Assume that the electro magnetic forces have therefore increased by 20% to 2.4e and 1.2e, respectively. As a result, the difference between the clockwise moment N1 and counterclockwise moment N2 about the center point O is N1xe2x88x92N2=xe2x88x922.4e. Therefore a counterclockwise moment N arises on the printed coil board 104. This moment N causes the moving member 100 to tilt in the radial direction (hereinafter is referred to as radial tilt). There is a problem that this radial tilt causes an aberration in the light beam spot focused on the recording face of the disc, adversely affecting the correct record and reproduction of signals.
An object of the present invention is to solve the above-mentioned problem associated with the prior art and to provide an objective lens driving apparatus having high driving sensitivities and capable of suppressing the occurrence of the radial tilt in the moving member.
An objective lens driving apparatus of the present invention comprises: an objective lens for focusing a light beam onto a recording face of a disc for recording and reproducing information on the disc; a lens holder for holding thereon the objective lens; a supporting member for supporting the lens holder so as to be movable in a focusing direction of an optical axis direction of the objective lens, and also in a tracking direction of a radial direction of the disc; a coil assembly attached to the lens holder, the coil assembly including a focusing coil and a tracking coil, each coil having a coiling axis oriented perpendicularly to a plane containing the focusing direction and the tracking direction; and a magnet assembly disposed opposite to the coil assembly, the magnet assembly including a first magnet, whose magnetic poles face to winding portions of the focusing coil receiving an electromagnetic force in the focusing direction (Fo) when a current flows through the coil assembly, and a second magnet, whose magnetic poles face to winding portions of the tracking coil receiving an electromagnetic force in the tracking direction.
According to the present invention, since the magnetic poles of the magnets face to all the winding portions on those sides of each tracking coil which are effective to produce a driving force, the driving force increases and a high driving sensitivity can be obtained in comparison with the prior art in which only a part of effective sides of each tracking coil faces to the magnetic pole. Furthermore, moments about the center of the focusing coil, caused by the electromagnetic forces acting on the winding portions on the sides of the tracking coils perpendicular to the focusing direction, i.e., the ineffective portions that do not contribute to the generation of the driving force are cancelled, thereby preventing the moving member from tilting in the radial direction of the disc, and making stable signal recording and reproducing operation under a high speed driving conditions.
An objective lens driving apparatus in another aspect of the present invention comprises: an objective lens for focusing a light beam onto a recording face of a disc for recording and reproducing information on the disc; a lens holder for holding thereon the objective lens; a supporting member for supporting the lens holder so as to be movable in a focusing direction of an optical axis direction of the objective lens, and also in a tracking direction of a radial direction of the disc; a coil assembly having a board attached to the lens holder, the coil assembly including a focusing coil formed in a center portion of the board and at least two tracking coils formed on both sides of the focusing coil on the board, wherein the tracking coils are arranged symmetrically to each other with respect to a center line extending across the focusing coil in parallel to the tracking direction and are aligned in a direction normal to the center line; and a magnet assembly including: a first magnet, which is disposed opposite to the focusing coil, and whose edge portion extending parallel to the focusing direction passes through the centers of the tracking coils so as to face to one-half region of each of the tracking coils, the first magnet having magnetic poles facing to winding portions of the focusing coil which extend in a direction intersecting the focusing direction; and a second magnet, which is disposed so as to face to the remaining one-half region of each of the tracking coils, and whose magnetic poles face to winding portions of the tracking coils which extend in a direction intersecting the tracking direction.
According to the invention of this aspect, the magnet poles opposite to the focusing coil and the tracking coils, all the coils being arranged in the same plane and substantially rectangular in shape, are magnetized so that their magnetic poles are formed at all the positions corresponding to those winding portions of the focusing coil and tracking coils which contribute to the generation of the driving forces in the respective driving directions. This serves to increase the driving sensitivities, increasing the tracking capability of the objective lens and making high speed recording and reproducing possible. Furthermore, the moments about the center point O of the printed coil board 4a, caused by the electromagnetic forces acting on the winding portions on the sides of the tracking coils extending perpendicularly to the focusing direction, i.e., the ineffective portions that do not contribute to the generation of the driving force, cancel each other, serving to suppress a radial tilt of the moving member. This makes stable recording and reproducing possible under high speed driving conditions. Moreover, by arranging a plurality of focusing coils in the tracking direction and changing the direction or the value of the current to be supplied to some of a plurality of focusing coils, it becomes possible to control the radial tilt.
An objective lens driving apparatus in another aspect of the present invention comprises: an objective lens for focusing a light beam on a disc for recording and reproducing information on said disc; a lens holder for holding said objective lens a supporting member for supporting said lens holder so as to be movable in a focusing direction along the optical axis of said objective lens, and in a tracking direction of a radial direction of said disc; a coil assembly formed on a board attached to said lens holder, said coil assembly including a focusing coil asymmetric with respect to a center line of said board in parallel to said focusing direction, and a tracking coil asymmetric with respect to said center line; and a magnet assembly including a first magnet having on a surface thereof two magnetic poles disposed opposite to said focusing coil, and a second magnet disposed adjacent to said first magnet on the same plane as said first magnet and having a boundary with said first magnet passing the centers of said tracking coils in the focusing direction.
According to the invention of this aspect, since the driving force is exerted on all the winding portions of two sides of each tracking coil, the acceleration sensitivity increases. As a result, the focusing and track-following ability of the objective lens increases, and high speed recording and reproducing operation is achieved. The moments about the center point, caused by the electromagnetic forces acting on the ineffective portions, i.e., the winding portions on the sides of the tracking coils perpendicular to the focusing direction, are cancelled with each other, serving to suppress the radial tilt of the moving member. Consequently, stable recording and reproducing operation is realizable under high speed driving. Since the focusing coil is asymmetric with respect to the center line, the focusing coil can be made so as to be longer in the tracking direction Tk. The area of the effective winding portions perpendicular to the focusing direction increases, and the area of the ineffective portions perpendicular to the tracking direction decreases. The proportion of the effective portions in the entire focusing coil increases.